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Sensing Systems and Signal Processing
Dr Richard
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Displacement – Encoders
There are many forms of encoders that utilise different operating principles for example conductive, optical, magnetic.
They can either give a signal related to the absolute position or a signal to show the position has changed (incremental) .
What applications are they used for?
Displacement – Encoders
Automotive: encoders used for sensing mechanical motion for speed control
Industrial: packing lines, machining, anything that moves that needs controlling!
Medical: medical scanners, dispensing pumps.
Instruments: sample positioning and scanning,
Consumer Electronics: printers, scanners,
Anywhere where something moves and the position / speed / direction is important.
Optical Encoder Example
The idea behind encoding transducers is that an (optical) pattern in the form of a binary code is printed on to or etched away from the moving component
This pattern is sensed by an (optical) reader and interpreted in terms of the displacement and direction of the moving part. Multiple track or detectors allow direction to be sensed.
The devices may be linear, measuring linear displacement, or circular, in which case they measure rotation
They can be designed to measure displacements as small as 1 micron, or rotations with resolutions of 1 in 20000 per revolution
2 PD 90 degree offset
2 PD tracks have 90 degree offset
Optical Encoder Example
A simple on off pattern does not give you absolute position only incremental motion. Which is fine for many applications.
Limit switches to ensure that the motion stage doesn’t go out of its working range are usually required.
You can make such a system give absolute position if you use limit switches and ‘home’ the stage.
Work out the full motion extent by looking for the two limit switches. Then go to one end of the travel and start a counter. The total number of counts for full motion and the current count give you absolute position.
2 PD 90 degree offset
2 PD tracks have 90 degree offset
Optical Encoder Example
For True absolute position the pattern on the object that is moving needs to be unique so that any output of the sensor can only be from one location.
This can be done by having multiple tracks on the linear rule or the rotating disk.
Each track has it’s own sensor, the fixed disk in the example below allows only light from the reference direction to be sampled by the photodiodes. Which photodiodes have light on uniquely identify the angle of the disk.
Optical Encoder Example
However, there is a problem with the binary code:
A very small error in geometry of the windows can produce erroneous transient states
At half the maximum rotation the output code changes 0111 to 1000
If the highest bit changes too quickly the output code will (momentarily) be 1111 i.e. 100% error!
Optical Encoder Example
The answer is to use a code in which only one bit changes at each counting step
The Gray code.
Decimal Binary Gray
0 000 000
1 001 001
2 010 011
3 011 010
4 100 110
5 101 111
6 110 101
7 111 100
Difference between two successive values.
Linear Encoder Example
Can use optical read heads or magnetic.
Incremental in their simplest form uses simple pattern of on-off to keep track of distance moved as described earlier
Absolute encoders require unique codes so that the absolute position can be known.
The optical image quality and speed of processing is very demanding in this case.
Allows very high positioning accuracy over a large range with high speed positioners.
https://resources.renishaw.com/en/details/installation-guide-resolute-rtla-and-fastrack-absolute-linear-encoder-system–102799
Incremental encoder
Absolute encoder
Magnetic Encoders
Magnetic Encoders use the same approaches as discussed for optical devices but they encode the patterns with North and South pole arrangements, these are commonly used in rotary encoders.
Detection is achieved either with a hall effect or magneto resistive detector.
Hall effect detector:
Sensitive to the magnetic deflection of electrons leading to a change in voltage across the sensor.
Size of voltage depends on materials used.
Used in analogue form to give an output voltage proportional to the magnetic field
Hall device
Magnetic Encoders
Or with Schmitt trigger to convert to digital signals
Devices are usually used in pairs to produce a quadrature signal (as is common in optical version discussed earlier).
Magnetic resistive sensors sense the change in resistance caused by a magnetic field.
Usually made from Permalloy a Nickle iron alloy, typical dR/R of 1-3% can be obtained
Usually used with whetstone bridge arrangements (we’ll cover these in future lecture)
Hall device
Digital out
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